I installed Microsoft Visual Studio 2015 Professional, however "mex -setup" still does not successfully find the Microsoft Visual C++ 2015 compiler. This can be seen from the following output snippet of running the command "mex -v -setup":

What we see in the output above is that indeed Microsoft Visual Studio 2015 has been installed, but the actual C/C++ compiler is missing. This is likely caused by having installed Microsoft Visual Studio 2015 Professional using the "Typical" setup option which will not install the Visual C/C++ compiler by default. This is seen in Microsoft's documentation: